netdev - Re: [PATCH net-next 06/12] net: dsa: rzn1-a5psw: add Renesas RZ/N1 advanced 5 port switch driver

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Message-ID: <20220415111419.twrlknxuto4pri63@skbuf>
Date:   Fri, 15 Apr 2022 14:14:19 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Clément Léger <clement.leger@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Magnus Damm <magnus.damm@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Herve Codina <herve.codina@...tlin.com>,
        Miquèl Raynal <miquel.raynal@...tlin.com>,
        Milan Stevanovic <milan.stevanovic@...com>,
        Jimmy Lalande <jimmy.lalande@...com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org, netdev@...r.kernel.org,
        Jean-Pierre Geslin <jean-pierre.geslin@....se.com>,
        Phil Edworthy <phil.edworthy@...esas.com>
Subject: Re: [PATCH net-next 06/12] net: dsa: rzn1-a5psw: add Renesas RZ/N1
 advanced 5 port switch driver

On Fri, Apr 15, 2022 at 12:02:14PM +0100, Russell King (Oracle) wrote:
> On Fri, Apr 15, 2022 at 01:55:03PM +0300, Vladimir Oltean wrote:
> > I meant that for a DSA switch driver is mandatory to call dsa_switch_shutdown()
> > from your ->shutdown method, otherwise subtle things break, sorry for being unclear.
> > 
> > Please blindly copy-paste the odd pattern that all other DSA drivers use
> > in ->shutdown and ->remove (with the platform_set_drvdata(dev, NULL) calls),
> > like a normal person :)
> 
> Those platform_set_drvdata(, NULL) calls should be killed - the
> driver model will set the driver data to NULL after ->remove has
> been called - so having drivers also setting the driver data to
> NULL is mere duplication.

I can see why you say that, but the reverse is not true.
A driver can be removed from a device after said device has been shut
down, and DSA does things in dsa_unregister_switch() and in
dsa_switch_shutdown() that are incompatible with each other, so either
one or the other should be called, but not both.
The platform_set_drvdata(dev, NULL) from the ->remove path may be
redundant for the reason you mentioned, but it doesn't really hurt
anything, really (it's a pointer assignment), and perhaps would lead to
even more confusion (why are we setting the drvdata to NULL from
->shutdown but not also from ->remove?).

> The only case it would matter is if someone is looking up the device
> and then accessing the driver data - and one would hope that's done
> with appropriate locking or other guarantees (e.g. driver can never
> be unbound once the driver data has been set.)

No, this isn't what is happening.